Filtering circuits are well-known in electronics and generally comprise inductive-capacitive circuits which are either arranged in series with input and output leads or shunt unwanted frequencies to ground. When a plurality of inductive elements are in close proximity, it is common for inductive coupling to occur. In some instances, this coupling is advantageously used to provide phase cancellation to increase the filtering capabilities of the circuit.
In cable television technology, notch filters are used to remove selected frequencies, such as those frequencies for which the subscriber has not paid. It is necessary in this art to provide at least 40 db attenuation, and this requires the use of phase cancellation techniques. Moreover, temperature and aging effects generally require filters to have at least 70 db attenuation.
To obtain greater than 40 db attenuation, it is known to provide a plurality of filters in series. At the frequencies generally employed in cable television, however, increases in attenuation are obtainable only if the filter sections are magnetically shielded from each other because of the effects of inductive coupling.
Thus, it is common to provide a first compartment having three inductive elements therein connected to a second compartment having identical inductors, wherein the two compartments are magnetically shielded from each other. Each pair of compartments filters out a single frequency, and a plurality of these units is connected together through coaxial cable connections to filter out the desired frequencies.